Apparatus for reactive distillation

ABSTRACT

The invention relates to a process and apparatus for manufacturing methyl tert-butyl ether (MTBE) by reacting methanol with isobutene-containing hydrocarbons in a reaction zone having alternate beds of sulfonated resin catalyst and catalyst-free distillation zones wherein the zones also contain liquid redistribution plates.

This is a division of application Ser. No. 171,340 filed Mar. 21, 1988now U.S. Pat. No. 4,847,431.

The present is a process for manufacturing a tertiary alkyl ether byreacting an aliphatic alcohol with a hydrocarbon mixture containing atleast one iso-olefin.

It also concerns an apparatus for carrying out said process.

The present invention more particularly concerns the manufacture ofmethyl tert-butyl ether (MTBE) from isobutene and methanol, themanufacture of tert-amyl methyl ether (TAME) from isopentene andmethanol and the manufacture of MTBE and TAME from isobutene, isopenteneand methanol.

BACKGROUND OF THE INVENTION

Tertiary alkyl ethers, particularly methyl tert-butyl ether (MTBE) andtert-amyl methyl ether (TAME) are of high interest for improving thegasoline qualities.

Thus, the use of methyl tert-butyl ether (MTBE),in view of its antiknockproperties, improves the quality of commercial gasolines, resulting in ahigher octane number than that obtained by addition of methanol, one ofthe best additives. In addition, methyl tert-butyl ether (MTBE) has acalorific value higher than that of methanol: 8,395 kcal/kg (35,091kJoule/kg) for MTBE, as compared with 4,764 kcal/kg (19,914 kJoule/kg)for methanol (as an average the calorific value of a primium gasoline is10,200 kcal/kg, i.e 42,636 kJoule/kg). Moreover, the use of MTBE doesnot result in demixion difficulties in the presence of water, as formethanol. Finally, the solubility in water of MTBE being considerablyhigher than that of water in hydrocarbons, the addition of MTBE improvesthe tolerance to water of motor-fuels.

It is known to prepare tertiary alkyl ethers, particularly methyltert-butyl ether (MTBE) and tert-amyl methyl ether (TAME) which are themost conventional ethers, by reacting an iso-olefin, generally containedin a hydrocarbon fraction, with an alcohol, for example methanol, in thepresence of an acid catalyst, for example sulfuric acid, hydrofluoricacid, aluminum chloride or boron fluoride, or in the presence ofcarbonaceous materials containing --SO₃ H groups, for example sulfonatedcoals, sulfonated phenol-formaldehyde resins, sulfonatedcoumarone-indene polymers or preferably sulfonatedpolystyrene-divinylbenzene resins.

It has been known for a long time that the reaction between methanol andtertiary olefins is a balanced reaction and therefore it is difficult toobtain high conversion rates. The equilibrium is less displaced in favorof ether formation so that the molecular weight of the iso-olefin ishigher. Thus, for iso-amylenes, the conversion rate is limited to 65-75%when the use of a too large methanol excess is to be avoided. To obtainacceptable conversion rates of iso-olefins, particularly ofiso-amylenes, a very large methanol excess with respect to thestoichiometry must be used. Consequently, the methanol, amount containedin the reaction mixture is too large to be easily removed in aconventional manner, such as by azeotropic distillation withhydrocarbons and recycling to the reactor, as disclosed in the Frenchpatent 2 411 881. According to U.S. Pat. No. 4,204,077, methanol couldthen be removed by extraction with a solvent such as ethylene glycol.

In these conventional techniques, the tertiary alkyl ether formed byreaction of an alcohol with an iso-olefin contained in a hydrocarbonmixture, is obtained within a mixture of unconverted hydrocarbons andoccasionally of unconverted alcohol. After completion of theetherification reaction, generally in at least two reactors, thetertiary alkyl ether must be separated from the other constituents inseveral distillation columns, while simultaneously removing the maximumalcohol amount with the minimum loss of ether.

Thus, the use of several reactors and distillation columns formanufacturing and separating the tertiary alkyl ether increases theinvestment and operating costs without giving a high ether yield.

A method has been proposed for solving the relevant problems: itinvolves in the reactive distillation (or catalytic distillation), inwhich the etherification reaction with a catalyst and the distillationfor separating the tertiary alkyl ether, as it is formed, from the otherunconverted constituents are performed in the same enclosure (U.S. Pat.No. 3,629,478 EP-B 8 860, FR 2 503 700).

According to U.S. Pat. No. 3,629,478, the catalyst is placed in bulk indischarge gutters (or downcomers) of distillation sieve trays: accordingto this patent only the descending liquid phase is in contact with thecatalyst, the vapor phase rising through the perforations of eachdistillation tray. In fact, due to the reaction exothermicity, theformation of a vapor phase at contact with the catalyst is unavoidableand gives rise to a hydrodynamic problem: as a matter of fact, it willbe very difficult, if not impossible, for a mixed phase consisting ofthe liquid plus the vapor formed by the reaction heat, i.e a lightphase, to descend through the discharge gutters, in view of the highresistance to its passage due to the small section of said gutters andto the catalyst contained therein.

The European patent 8 860 proposes to feed an isobutene andmethanol-containing mixture into a distillation column filled with acatalyst convenient for producing methyl tert-butyl ether (MTBE),wherein the catalyst also acts as packing for the distillation, thusforming MTBE and simultaneously separating C₄ constituents.

Although the process disclosed in this patent already represents animportant technical advance in the field of reactive distillation, it isfurther substantially improved according to the invention, as a result,in particular, of a more important distillation operation.

French patent 2 503 700 proposes the use of a series of catalytic stepswith ascending vapor-liquid flow through each catalyst bed with thecatalyst being embedded. But the distillation effect is not as importantas expected. Moreover, a hydrodynamic problem may arise : as a matter offact, in view of the gravity effect, it is not easy for the fluid toflow upwardly through each catalyst bed.

SUMMARY OF THE INVENTION

The present invention copes with the above-mentioned disadvantages bysimultaneously performing in the same single enclosure, in the presenceof a suitable catalyst, the reaction producing the tertiary alkyl etherand the separation of the latter, by distillation, from the accompanyinghydrocarbons and compounds, while obtaining very satisfactory yields toether, mainly as a result of a very good distillation effect.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be now described more in detail by way ofillustrative and non-limiting examples, with reference to theaccompanying drawings, wherein :

FIGS. 1, 2 and 3 illustrate the enclosure according to the invention,FIG. 1 being a view of the middle part of the enclosure, FIGS. 2 and 3being respectively longitudinal cross-sectional views of the upper partand of the bottom part of the enclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

One object of the present invention is to provide a process formanufacturing one or more tertiary alkyl ether(s), by reacting analiphatic alcohol with a hydrocarbon mixture containing at least oneiso-olefin, in a reaction-distillation zone (where the reaction and/orthe distillation are performed) defined (see FIGS. 1, 2 and 3) by anenclosure (1) of substantially cylindrical shape, for example vertical,containing at least one catalyst of the sulfonated resin type, forexample a sulfonated polystyrene-divinylbenzene resin, said processbeing characterized by the steps of:

i - introducing the charge of reactants, comprising at least saidalcohol and at least said hydrocarbon mixture, into saidreaction-distillation zone which contains:

(a) at least two superposed and non-contiguous fixed beds (2a, 2b) ofsaid catalyst of sulfonated resin type, each of said beds being placedon a perforated support member or base plate (3a,3b) (which may be forexample a perforated disc, a supporting grid or a net) and comprising aplurality of fabric pockets containing said catalyst, said pocketsleaving free spaces therebetween, said catalyst beds being optionally ofdifferent thicknesses,

(b) at least one catalyst-free distillation zone in at least one freespace between two superposed and non-contiguous catalyst beds (2a,2b),

(c) at least one catalyst-free discontinuous tray for liquidredistribution (5) (i.e, provided with a plurality of discontinuities),located in at least one free space between a distillation zone and thecatalyst bed just below said distillation zone,

ii - maintaining the distillation conditions in saidreaction-distillation zone, so as to obtain a descending liquid phaseand an ascending vapor phase in said zone, iii - discharging from thetop (line 8) of the reaction- distillation zone a vapor phase mainlycontaining unconverted hydrocarbons and,

iv - withdrawing from the bottom (10) of the reaction-distillation zonea liquid phase mainly containing said tertiary alkyl ether(s).

The term distillation zone is used to designate any device known in theart for performing a distillation, such as:

at least one inert packing with a support member, and/or

at least one discontinuous distillation tray (4a) (i.e, a tray providedwith discontinuities for giving passage to the vapor phase, and with atleast one discharge duct (or downcomer) edged with an overflow for theliquid phase), mainly selected from the following type:

sieve trays,

valve trays,

bubble-cap trays.

Each liquid redistribution tray (5) is provided with a plurality ofdiscontinuities. At least one of said discontinuities consists of afunnel or duct (12) for the passage of the vapor phase, the upper endpart of which is preferably protruding, i.e raises above the bottom ofsaid liquid redistribution tray (5), said end being preferably covered(but not closed) e.g., by a cap (12a), thus preventing the liquidpresent on said tray from being discharged through said funnel or duct.Other discontinuities of said tray are orifices for the passage of theliquid phase, these orifices being so distributed as to make possible auniform spraying with liquid phase of the catalyst bed just below eachliquid redistribution tray.

In each of the catalyst beds, the free space around the fabric pocketscontaining the catalyst (said pockets being permeable to liquid butimpermeable to solid catalyst particles) give free passage to liquid andvapor, hence with a certain distillation effect, substantially improvedby the presence of one or more distillation zones.

Advantageously, at least a part of the vapor phase discharged from thetop (line 8) of the reaction-distillation zone may be condensed (in acondenser external to the enclosure (1) defining said zone, not shown inthe figures), then fed back to said zone, for example to the top thereof(line 9), as liquid flow, called reflux.

Similarly, at least a part of the liquid phase withdrawn from the bottomof the reaction-distillation zone through line (10) may be vaporized (bypassage through a reboiler external to enclosure 1, not shown in thefigures) and then reintroduced into said zone, for example at the bottompart thereof, through line (11), as vapor flow, called reboiling vapor.

According to a preferred embodiment of the invention (see FIG. 1) whereat least one distillation zone comprises one or more distillation trays,the liquid phase is supplied onto each distillation tray just below acatalyst bed, at the most remote place from the overflow thereof, i.e,at the side opposite the overflow, after previous passage over acatalyst-free liquid distribution tray (6), traversed by at least oneduct or funnel (13) for the passage of the vapor phase (duct or funnelwhose upper end is preferably protruding above the tray bottom, saidupper end being provided with a cap (13a) for preventing the liquid flowpassage through said duct or funnel), substantially inclined, located ineach free space between a distillation tray (4a) and the catalyst bedjust above said tray, each liquid distribution tray (6) being furtherprovided, at its lowermost end, with a free passage-way (14) (preferablyconsisting of a discharge gutter or duct edged with a small lip) for theliquid phase: the liquid downward flow from the distillation tray isthus more regular and the distillation on said tray still moreefficient.

The space between the top of the reaction-distillation zone (i.e, thetop of the enclosure) and the uppermost catalyst bed of said zone (seeFIG. 2) may preferably contain a catalyst-free distillation zone (forexample, at least one discontinuous distillation tray 4b).

The space between the bottom of the reaction-distillation zone (thebottom of the enclosure) and the lowermost catalyst bed of said zone(see FIG. 3), may also preferably contain a catalyst-free distillationzone (for example at least one discontinuous distillation tray 4c).

According to another preferred embodiment of the invention (see FIG. 1),the charge of reactants, containing at least one aliphatic alcohol andat least one hydrocarbon mixture, is introduced into thereaction-distillation zone at a level (line 7) thereof below at leastone catalyst bed and more preferably such that the lowermost catalystbed of said zone is above said level.

It is optionally possible, in addition to the charge, to introduce saidalcohol separately (i.e alone) into the reaction-distillation zone,through at least one inlet port different from that of said charge andpreferably located in the vicinity of the top of saidreaction-distillation zone (for example, above the uppermost catalystbed of said zone and preferentially below the reflux feeding port).

This additional alcohol amount favors the etherification reaction andresults in a higher conversion rate of the iso-olefin. Moreover, it hasthe effect of reducing the eventual formation of dimers. The alcohol ispreferably injected at a temperature lower than its boiling temperature.

According to another embodiment of the invention, the fabric pocketscontaining the catalyst may be fastened to grids which are rolled up onthemselves and superposed on each other and whose meshes are in majorpart metallic (each catalyst bed thus comprising at least one layer ofsaid grids).

The reflux ratio (i.e, a ratio between the reflux liquid and thewithdrawn liquid),in proportion to the distillate, is generallymaintained in the range from 0.1:1 to 20:1, preferably from 0.5:1 to10:1. The operation is mostly conducted inside enclosure (1) within arelatively wide pressure and temperature range:for example, a pressureof 1-30 bars (100-3,000 kPa), preferably 2-20 bars (200-2000 kPa) and atemperature from 10° to 200° C., preferably from 40° to 160° C,, in thewhole enclosure.

Each of the catalyst beds used according to the invention fills thewhole circular section of the reaction-distillation zone, i.e, the wholecircular section of enclosure (1).

In FIG. 1, given by way of example, the liquid phase preferentiallyfollows the path indicated by arrows 20 and the vapor phasepreferentially follows the path indicated by arrows 30.

As it flows downwardly through the reaction-distillation zone, theliquid phase progressively increases its content of tertiary alkyl etherwhich is less volatile than the alcohol and the still unreactediso-olefin, and than the other non etherifiable constituents of thehydrocarbon mixture which in contrast to the tertiary alkyl ether, tendto flow upwardly towards the top of the reaction-distillation zone. Thusthe liquid phase withdrawn at the bottom of said zone mainly containstertiary alkyl ether. Moreover, the vapor phase discharged from the topof said zone is generally free of unconverted alcohol, except as traces.

Another object of the present invention (see FIGS. 2 and 3) is toprovide an apparatus or vessel (1) substantially of cylindrical shape,comprising at least one line (7) for introducing a charge, at least oneline (8), at the top of said vessel, for withdrawing a vapor phase, atleast one line (9),in the vicinity of the top of said vessel, for areflux feed, at least one line (10), at the bottom of said vessel, forwithdrawing a liquid phase, at least one line (11) for introducing an atleast partly vaporized phase, said vessel being characterized in that itcontains:

(a) at least two superposed and non-contiguous fixed catalyst beds (2a,2b), each of said beds, being placed on a perforated support member orbase plate (3a, 3b) (consisting for example of a perforated disc, asupporting grid or a net), filling the whole internal circular sectionenclosure (1),

(b) at least one distillation device, located in at least one free spacebetween two superposed and non-contiguous catalyst beds (2a, 2b), and

(c) at least one discontinuous liquid redistribution tray (5) (providedwith a plurality of discontinuities such for example as ducts or funnelswhose upper end may be protruding and covered), for liquidredistribution, located in at least one free space between adistillation apparatus and the catalyst bed just below it.

By distillation apparatus, it is meant any apparatus known in the artfor performing a distillation, such as:

at least one inert packing provided with a support member, and/or

at least one discontinuous distillation tray (4a) (i.e, provided withdiscontinuities for the passage of the vapor phase and with at least onedischarge duct (or downcomer) edged with an overflow for the liquidphase) mainly selected from the following types:

sieve trays,

valve trays,

bubble-cap trays.

The enclosure (1) according to the invention may further comprise, whenat least one distillation device is formed of one or more distillationtrays, at least one liquid distribution tray (6), wherethrough passes atleast one duct or funnel (whose upper end is preferably protruding andprovided with a cover), substantially inclined, located in at least onefree space between a distillation tray and the catalyst bed just aboveit, each liquid distribution tray being provided at its lowermost endwith at least one free passage-way (preferably consisting of a dischargegutter or duct edged with an overflow).

The enclosure (1) according to the invention may also contain at leastone device selected from the following group of:

at least one distillation device between the top of the vessel and theuppermost bed therein, and

at least one distillation device between the bottom of the vessel andthe lowermost catalyst bed therein.

This apparatus is adapted for carrying out the process according to theinvention.

The apparatus may be used, for example, for manufacturing methyltert-butyl ether (MTBE) from methanol and isobutene, for manufacturingtert-amyl methyl ether (TAME) from methanol and isopentene andmanufacturing MTBE or TAME from methanol, isobutene and isopentene, inthe presence of a convenient catalyst such as a catalyst of sulfonatedresin type (for example a sulfonated polystyrene-divinylbenzene resin),these types of use being not limitative.

EXAMPLE (comparative)

A charge, formed of methanol and of a mixture of butenes and butanescontaining about 25% of isobutene, already converted to MTBE in aproportion of 80% over a bed of sulfonated resin catalyst, is introducedinto an enclosure containing a plurality of beds of said catalyst and ofdistillation trays : according to the process of the invention(operating pressure of about 10 bars, temperature ranging from about 60°to 135° C. and reflux ratio of about 1:1) about 80% of the residualisobutene may then be converted to MTBE in said enclosure, in particularby placing a distillation tray in each space available between twoconsecutive catalyst beds. It is observed that, for the same results,the number of said catalyst beds used in the process of the invention isone half of the number of such beds required in a conventional catalyticcolumn.

What is claimed as the invention is:
 1. An apparatus for reactivedistillation comprising:a substantially cylindrical vessel having anupper portion and a lower portion, at least one feed inlet means forintroducing a charge, at least one vapor outlet means in said upperportion of said vessel for withdrawing a vapor phase, at least onereflux inlet means in said upper portion of said vessel for a refluxfeed, at least one liquid outlet means in said lower portion of saidvessel for withdrawing a liquid phase, at least one vapor inlet means insaid lower portion of said vessel for introducing an at least partiallyvaporized phase; at least two superposed non-contiguous fixed catalystbeds positioned within said vessel, each of said beds being placed on aperforated support member, at least one distillation means placed in atleast one free space between two consecutive catalyst beds, saiddistillation means having means to permit an upwardly flowing vaporstream to come into contact with a downwardly flowing liquid stream; andat least one discontinuous liquid redistribution tray, placed in atleast one free space between a distillation means and the catalyst bedjust below said distillation means to distribute liquid onto saidcatalyst bed wherein said catalyst bed, distillation means, andredistribution tray are positioned to direct a downwardly flowing liquiddischarged from said distillation means onto and through saidredistribution tray and then to flow downwardly through said catalystbed.
 2. An apparatus according to claim 1, further comprising at leastone distillation means having means to permit an upwardly flowing vaporstream to come into contact with a downwardly flowing liquid stream,said distillation means being positioned above the uppermost catalystbed of said vessel.
 3. An apparatus according to claim 1, furthercomprising at least one distillation means, having means to permit anupwardly flowing vapor stream to come into contact with a downwardlyflowing liquid stream, said distillation means being positioned belowthe lowermost catalyst bed of said vessel.
 4. An apparatus according toclaim 2, further comprising at least one distillation means, havingmeans to permit an upwardly flowing vapor stream to come into contactwith a downwardly flowing liquid stream, said distillation means beingpositioned below the lowermost catalyst bed of said vessel.
 5. Anapparatus according to claim 2, wherein said distillation meanspositioned above the uppermost catalyst bed is a discontinuousdistillation tray.
 6. An apparatus according to claim 3, wherein saiddistillation means positioned below the lowermost catalyst bed is adiscontinuous distillation tray.
 7. An apparatus according to claim 4,wherein said distillation means positioned above the uppermost catalystbed is a discontinuous distillation tray.
 8. An apparatus according toclaim 1, wherein said discontinuous liquid redistribution tray isprovided with at least one duct to permit the upward flow of a vaporstream, said duct having a top portion to which a cap means is attached,said cap means inhibiting the flow of liquid down said duct.
 9. Anapparatus according to claim 1, further comprising at least onesubstantially inclined liquid distribution tray, said liquiddistribution tray being located in at least one free space between saiddistillation means and said catalyst bed and being positioned above saiddistillation means, said liquid distribution tray having at least oneduct to permit the upward flow of a vapor stream and said liquiddistribution tray being provided at its lowermost end with at least onefree passage-way to permit the discharge of a downwardly flowing liquidfrom said liquid distribution tray and onto said distillation means. 10.An apparatus according to claim 1, wherein said feed inlet means ispositioned below said at least one catalyst bed.
 11. An apparatusaccording to claim 1, wherein said feed inlet means is positioned belowthe lowermost catalyst bed of said vessel.